The invention relates to a method of storing and transporting electrochemical energy using an electrochemical power station based on alkali metal, in particular sodium, and sulfur, in which the two reactants of the electrochemical reaction flow from stock containers into an electrochemical reactor and the products are discharged after the electrochemical reaction.
The generation of electrochemical energy is, in the case of fossil fuel power stations, associated with the production of CO2 and therefore has a considerable influence on the greenhouse effect. Generation of energy on the basis of renewable energy carriers, e.g. wind, solar, geothermal or hydroelectric, avoids this disadvantage. However, these renewable energy carriers are at present not available at any time to match the required load. In addition, the place of energy generation may differ from the site where the energy is required. To compensate this disadvantage inherent in the system, storage, buffering and, if appropriate, also transport of the energy generated is necessary.
The energy from renewable sources such as wind turbines, solar plants is not obtained continuously. Demand and availability are not matched. A power grid which is based exclusively on renewable energy sources and is nevertheless stable cannot exist under these boundary conditions. There is a need to equalize and buffer these fluctuations with high efficiency by means of inexpensive and energy-efficient systems.
In many sparsely populated regions of the earth, e.g. the Sahara, Iceland or offshore, there is the potential to generate electric power extremely efficiently from wind, sun or geothermal energy via wind turbines, solar plants or geothermal power stations owing to the geographic, climatic and geological boundary conditions. However, there is today a lack of industrial methods of transporting this energy to regions having a high consumption. Traditional transport systems are limited by grid losses and costs of grid construction. Hydrogen technology, in which electric energy generated on site is converted into hydrogen and subsequently converted into electric power in a fuel cell has an overall efficiency of about 20% and is thus unattractive since transport and liquefaction of the hydrogen consume a large proportion of the energy.
The storage of large quantities of electric energy is, like the transport of electric energy over large distances, a problem which has been solved only unsatisfactorily up to the present. Pump storage power stations in which the potential energy of the geodatic height difference of water is utilized for conversion into electric power are at present used for storing electric energy on an industrial scale. However, the construction of such pump storage power stations is limited by geographical and ecological boundary conditions. Pressure storage power stations in which the compression of air is used for energy storage are limited because of their comparatively low efficiency. Other forms of energy storage such as supercapacitors or flywheels also address other target markets (short-term storage). Batteries, for which various concepts have been realized industrially, come closest to this requirement.
DE-A-2635900 discloses a battery which comprises at least one molten alkali metal as anode and a cathodic reaction participant which is capable of a reversible electrochemical reaction with the anodic reaction participant. The cathodic reaction participant comprises molten polysulfide salts or a two-phase mixture of molten sulfur and polysulfide salts saturated with molten sulfur. Furthermore, this battery has cation-permeable barrier layers for liquid transport between the anodic reaction zone and the cathodic reaction zone.
DE-A-2610222 discloses a battery comprising a plurality of sulfur-sodium cells, in which each cell has a cathodic compartment comprising a cathodic reactant which is liquid at operating temperature and is composed of sulfur, phosphorus or selenium or alkaline salts of these elements, at least one solid electrolyte tube comprising an anodic reactant which is liquid at the operating temperature and is composed of an alkali metal, in particular sodium, and also an anodic vessel which comprises a reserve of the anodic reactant.
Connecting a plurality of sodium-sulfur batteries as module for an energy storage system is known from EP-A-116690.
It is common to all these batteries that, as closed systems, their energy is limited by the amount of reactants (redox partners) comprised in the battery. This limitation was overcome by the flow battery. The basis of this battery concept is formed by liquid electrolytes comprising solvent and metal salt. The limited reservoir volume of the classical battery is increased by second stock containers comprising the reactants.
DE-A-2927868 discloses a flow battery for the storage and liberation of electric energy in an electrochemical cell having an anode compartment and a cathode compartment which are separated from one another by a semipermeable ion-exchange membrane, with the anode compartment being supplied with an anolyte solution, an oxidizable compound which essentially remains dissolved in the anolyte solution and can be reduced again from its oxidized form, the oxidized anolyte solution is removed from the anolyte compartment and the oxidized anolyte solution stored. At the same time, the catholyte compartment is supplied with a catholyte solution, a reducible compound which essentially remains dissolved in the catholyte solvent and can be reoxidized from its reduced form. The anolyte solution and the catholyte solution can be stored in two corresponding vessels and be circulated through the anode and cathode compartments by means of circulating pumps. The catholyte solution can, for example, comprise hexavalent bromine and the anolyte solution can comprise divalent bromine.
DE-A-1771148 and U.S. Pat. No. 3,533,848 disclose a system for obtaining electric energy by means of electrochemical combination of sodium and sulfur, which comprises a diaphragm through which sodium ions can pass having adjacent spaces for sodium and sulfur, a container for storing the sodium outside the cell, lines for conveying the sodium from the storage container to the fuel cell, a container for storing the sulfur outside the cells and lines for conveying the sulfur from the storage container to the cell. These cells can, for example, be electrically connected in series.
JP 2001/118598 discloses operating sodium-sulfur batteries using two or more tanks for molten sodium.
JP-A-200284456 discloses operating a sodium-sulfur battery using an external storage tank for sulfur which is connected in a fixed manner to the battery.
In the case of the known sodium-sulfur batteries and their embodiments as flow battery, the introduction of the energy stored in the starting materials sodium and sulfur and discharge by reaction of sodium and sulfur to form sodium sulfide or sodium polysulfides are coupled both in time and in location.